Newt Gingrich has been warning the nation of the danger of an electromagnetic pulse (EMP)—a burst of radiation created by a high-altitude nuclear explosion. This pulse would take down electrical systems over hundreds or thousands of miles, the argument goes, knocking the U.S. back to the 19th Century. “In theory, a relatively small device over Omaha would knock out about half the electricity generated in the United States,” he was quoted as saying by the New York Times. In Gingrich’s view, the threat of an EMP attack justifies actions such as pre-emptive strikes on the missile instillations of nations such as Iran and North Korea.

The threat of an EMP attack is real—assuming, of course, that a nation or organization develops not only nuclear weapons but intercontinental ballistic missiles and the will to deploy them. Yet the primary target of an EMP wouldn’t be ground-based power systems. It would be satellites.

In the June 2004 issue of Scientific American, the national security journalist Daniel G. Dupont wrote “Nuclear Explosions in Orbit” [subscription required], a story that details the sequence of events that would follow a nuclear detonation just above the atmosphere.

The initial blast of high-energy gamma rays would strike air molecules and create a shower of high-energy electrons, he writes. These electrons, once they reached ground, would indeed disrupt sensitive electronic equipment. But only that equipment within direct line-of-sight of the blast—taking out a city, perhaps, not a continent.

More fearsome would be the effects of radiation on orbiting satellites. After the initial nanoseconds-long blast of gamma rays, a nuclear bomb releases about 70 percent of its total energy in the form of x-rays. Dupont writes:

“Soft,” or low-energy, x-rays produced by a HANE would not penetrate deeply into any spacecraft they encountered. Instead they would generate extreme heat at the outer surfaces, which itself could harm the sophisticated electronics inside. Soft x-rays would also degrade solar cells, impairing a satellite’s ability to generate power, as well as damaging sensor or telescope apertures. When high-energy x-rays strike a satellite or other system components, however, they create strong internal electron fluxes that produce strong currents and high voltages that can fry sensitive electronic circuitry.

He quotes K. Dennis Papadopoulos, a plasma physicist at the University of Maryland who studies the effects of high-altitude nuclear explosions for the U.S. government, who concludes that “a 10-kiloton nuclear device set off at the right height would lead to the loss of 90 percent of all low-earth-orbit satellites within a month.” The exception would be U.S. military satellites, many of which have been hardened against exactly this kind of threat.